linux-kernel - Re: 2.6.31 under "heavy" NFS load.

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Message-ID: <4AF9B994.8040301@krogh.cc>
Date:	Tue, 10 Nov 2009 20:05:56 +0100
From:	Jesper Krogh <jesper@...gh.cc>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	Greg Banks <gnb@...h.org>
Subject: Re: 2.6.31 under "heavy" NFS load.

J. Bruce Fields wrote:
> On Mon, Nov 09, 2009 at 08:30:44PM +0100, Jesper Krogh wrote:
>> When a lot (~60 all on 1GbitE) of NFS clients are hitting an NFS server
>> that has an 10GbitE NIC sitting on it I'm seeing high IO-wait load
>> (>50%) and load number over 100 on the server. This is a change since
>> 2.6.29 where the IO-wait load under similar workload was less than 10%.
>>
>> The system has 16 Opteron cores.
>>
>> All data the NFS-clients are reading are "memory recident" since they
>> are all reading off the same 10GB of data and the server has 32GB of
>> main memory dedicated to nothing else than serving NFS.
>>
>> A snapshot of top looks like this:
>> http://krogh.cc/~jesper/top-hest-2.6.31.txt
>>
>> The load is generally alot higher than on 2.6.29 and it "explodes" to
>> over 100 when a few processes begin utillizing the disk while serving
>> files over NFS. "dstat" reports a read-out of 10-20MB/s from disk which
>> is close to what I'd expect. and the system delivers around 600-800MB/s
>> over the NIC in this workload.
> 
> Is that the bandwidth you get with 2.6.31, with 2.6.29, or with both?

Without being able to be fully accurate, I have a strong feeling that
the comparative numbers on 2.6.29 were more around 800-1000MB/s. But
this isn't based on any measurements so dont put too much into it. I'll
try to make up something that I can use for testing over multiple
kernel-versions.

> Are you just noticing a change in the statistics, or are there concrete
> changes in the performance of the server?

Interactivity on the console is alot worse. Still usable, but top takes
~5s to start up on 2.6.31 where I didn't remember any lags on 2.6.29 (so
less than 2s).

>> Sorry that I cannot be more specific, I can answer questions on a
>> running 2.6.31 kernel, but I cannot reboot the system back to 2.6.29
>> just to test since the system is "in production". I tried 2.6.30 and it
>> has the same pattern as 2.6.31, so based on that fragile evidence the
>> change should be found in between 2.6.29 and 2.6.30. I hope a "wague"
>> report is better than none.
> 
> Can you test whether this helps?

I'll schedule testing..

-- 
Jesper
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